(a) Field of the Invention
The present invention relates to an image forming apparatus for use in a printer, a copying machine, a facsimile machine and the like and, more specifically, to a curl correcting unit for effecting curl correction as well as to an image forming apparatus provided with such a curl correcting unit.
(b) Description of the Prior Art
In general, an image forming apparatus using a xerographic system, such as a laser printer or a color image copying machine, makes use of means which is arranged to expose, by laser or the like, an image carrier such as a photoconductor drum to image data inputted from a personal computer or an image input terminal (IIT) and form an electrostatic latent image on the image carrier, develop the electrostatic latent image with toner by a developing unit and transfer the toner image to a sheet (mainly, paper), fix the image by fusing the toner to the sheet by a heating roller type of fixing unit, and deliver the fixed sheet to the outside of the apparatus. In the field of such an image forming apparatus, there has been provided an apparatus of the type in which a unit for executing postprocessing such as stapling, punching and sorting is connected to an image forming apparatus for the purpose of realizing automation of postprocessing work. It has heretofore been known that, in the use of such a postprocessing unit, the state of curling of a sheet deeply concerns various factors such as the occurrence of a jam due to a failure in sheet transportation, a success or a failure in sheet registration in a sorter or the like and the level of postprocessing performance.
Since the curl of a sheet varies according to the kind of sheet, temperature, humidity and the like, there already exists an art which detects humidity, the moisture content of a sheet, the kind of sheet and sheet thickness and adjusts a curl correction quantity, as disclosed in, for example, Japanese Patent Laid-Open No. 251067/1992. In addition, although toner transferred to a sheet is heated for fixation and, after delivered from the postprocessing unit, shrinks with a decrease in temperature, this shrinkage greatly affects the curl of the sheet. In view of this fact, for example, Japanese Patent laid-Open Nos. 217313/1996 and 30712/1997 discloses the art of adjusting a curl correction quantity according to the image density or the detection of result of the amount of toner.
In the art disclosed in any of these specifications, curl correcting parts for effecting different corrections on the basis of the curl correction quantity determined in the above-described manner are disposed along different sheet transporting paths, and the transporting path of a sheet is selectively switched over, thereby effecting curl correction on the sheet. One of the curl correcting parts is provided for effecting the correction of curving a down curl, which acts to lower the edge portions of a sheet, upwardly (outwardly in the downward directions on the basis of the curl correction quantity, while the other curl correcting part is provided for effecting the correction of curving an up curl, which acts to raise the edge portions of a sheet, downwardly (outwardly in the upward direction) on the basis of the curl correction quantity.
Japanese Patent Laid-Open No. 48343/1981 discloses the art of detecting the curling direction of a sheet and carrying out correction of a down curl or an up curl by means of curl correcting parts which are sequentially arranged along an identical path. In this art, solenoids are individually disposed in the respective curl correcting parts, and the solenoids are alternatively operated to correct and flatten a sheet, thereby effecting curl correction.
However, it is in general difficult to reliably detect the state of a curl-corrected sheet and newly apply curl correction to the sheet, and a certain extent of curl remains even if settings which aim at flattening or straightening the sheet are adopted. In this case, for example, if a sheet is delivered to a delivery tray with an up curl remaining in the sheet, as shown in FIG. 15(a), the previous delivered sheet covers a delivery portion and blocks the next sheet to be delivered, thus leading to a stacking failure in which the previous delivered sheet rolls and the next delivered sheet covers the rolled sheet, or a delivery failure in which the next sheet fails to be reliably delivered. In particular, if a multiplicity of sheets are stacked on a delivery tray, even a small up curl will eventually cause such a trouble. On the other hand, if a sheet is delivered in a down-curled state as shown in FIG. 15(b), the occurrence of any of the above-described troubles can be prevented.
Such a problem conspicuously appears when a sheet passes through only a curl correcting part which is disposed along one of sheet transporting paths to carry out correction of a down curl, as disclosed in Japanese Patent Laid-Open Nos. 251067/1992, 217313/1996 and 30712/1997. In addition, even if one curl correcting part is to be alternatively selected from curl correcting parts which are sequentially arranged along an identical sheet transporting path, as disclosed in Japanese Patent Laid-Open No. 48343/1981, a similar trouble will occur when only the correction of a down curl is selected or an up curl is insufficiently corrected.
In addition, if a down-curl correcting part and an up-curl correcting part are sequentially arranged in a sheet transporting direction and are driven by an identical driving source, the driving source can be made simple and a great reduction in cost can be achieved.
In Japanese Patent Laid-Open No. 48343/1981, an arbitrary curl correcting part is selected from among the curl correcting parts by means of different driving sources. However, if this art is developed to drive the curl correcting parts by an identical driving source, there may be a case in which the switchover of the curl correcting parts cannot follow the speed of sheet transportation and an unexpected curl correction may be performed on the next sheet to be delivered, by the curl correcting part for the previous delivered sheet. In such a case, if curl correction is performed in a direction in which a sheet is curved outward in the upward direction (the edge portions of the sheet are lowered), no serious problem occurs, but if curl correction is unexpectedly performed in a direction in which a sheet is curved outward in the downward direction (the edge portions of the sheet are raised), the above-described delivery troubles will occur.
Accordingly, if the down-curl correcting part and the up-curl correcting part are sequentially arranged in the sheet transporting direction, it is necessary to take measures so that, even in the worst case, a sheet can be delivered in the state of being curved outward in the upward direction (with the edge portions of the sheet being lowered).
In addition, the speed of sheet transportation is increased in order to improve the productivity of the printing speed of the image forming apparatus, and the space between sheets being transported is designed to be extremely small in order to realize a reduction in the total time period required to continuously record images an sheets.
In the case in which the down-curl correcting part and the up-curl correcting part are sequentially arranged in the sheet transporting direction, if the space between both curl correcting parts is larger than the space between sheets being continuously transported, there occurs a state in which at the same time that the previous delivered sheet is present at one of the curl correcting parts, the next sheet to be delivered is present at the other.
In this case, if the respective curl correcting parts can be quickly switched over by different driving units, no problem will occur. However, if both curl correcting parts are to be driven by an identical driving source, curl correction will have to be abandoned at either one of the curl correcting parts.
In this case, if curl correction is performed on a sheet in a direction in which the sheet is curved outward in the upward direction (in a direction in which the edge portions of the sheet are lowered), no problem will occur. However, in a case where curl correction needs to be performed on a sheet in a direction to curve the sheet outward in the upward direction (in a direction to lower the edge portions of the sheet), if curl correction is performed on the sheet in the opposite direction, an extremely serious trouble will occur during the delivery of the sheet.